Metal oxide material

ABSTRACT

The invention provides certain novel metal oxide materials which exhibit superconductivity at elevated temperatures and/or which are useful in electrode, electrolyte, cell and sensor applications, or as electrochemical catalysts. The metal oxide materials are generally within the formula 
     R n+1−u−s A u M m+e Cu n O w   ( 1 ) 
     where n≧0 and n is an integer or a non-integer, 1≦m≦2, 0≦s≦0.4, 0≦e≦4, and 2n+(1/2)&lt;w&lt;(5/2)n+4, with the provisos that u is 2 for n≧1, u is n+1 for 0≦n&lt;1  
     and where  
     R and A are each any of or any combination of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Ca, Sr, Ba, Li, Na, K, Rb or Cs; M is any of or any combination of Cu, Bi, Sb, Pb, Tl or any other transition metal; Cu is Cu or Cu partially substituted by any of or any combination of Bi, Sb, Pb, Tl or any other transition metal; O is 0 or O partially substituted by any of N, P, S, Se, or F; and wherein the structure of the materials is characterised by distorted or undistorted substantially square planar sheets of CuO 2  when n&gt;0 and distorted or undistorted substantially square sheets of R for n&gt;1.

[0001] The invention comprises certain novel metal oxide materials whichexhibit superconductivity at elevated temperatures and/or which areuseful as electrodes or electrolytes in electrochemical cells, sensors,and as catalysts.

[0002] It is known that certain classes of metal oxide will exhibit thephenomenon of superconductivity below a particular critical temperaturereferred to as T_(c). These include as prototypesBaPb_(2−x)Bi_(x)O_(3−d), Ba_(2−x)Sr_(x)CuO_(4−d), YBa₂Cu₃O_(7−d) asdescribed in The Chemistry of High Temperature Superconductors, ed. byNelson et al, American Chem. Soc. 1987, and Bi₂Sr₂CaCu₃O_(5−d) asdescribed by Subramanian et al, Science 239, 1015 (1988). We haveidentified this last material as the n=2 member in a homologous seriesof approximate formula Bi₂(Sr,Ca)_(n+1)Cu_(n)O_(2n+4+d), n=0,1,2,3, . .. , obtained by inserting an additional layer of Ca and an additionalsquare planar layer of CuO₂ in order to obtain each higher member. Thesematerials often exhibit intergrowth structures deriving from a number ofthese homologues as well as Bi substitution on the Sr and Ca sites.T_(c) is observed to rise as n increases from 1 to 2 to 3. The materialYBa₂Cu₄O_(8+d) has a layered structure similar to the n=2 member of thisseries Bi₂Sr₂CaCu₂O_(8+d) and we expect therefore that YBa₂Cu₄O_(8−d)belongs to similar series. One such seril es could be obtained byinsertion of extra Y—CuO₂ layers resulting in the series of materialsR_(n)Ba₂Cu_(n+3)O_(3.5+2.5n−d), n=1,2,3, . . . and another by insertionof extra Ca—CuO₂ layers resulting in the seriesRBa₂Ca_(n)Cu_(n+4)O_(8+2n−d), n=1,2, . . . By analogy it may be expectedthat T_(c) in these two series should rise with the value of n.

[0003] Binary, ternary or higher metal oxide materials containing ascations one or more alkali earth elements, such as these materials andhaving high oxygen-ion mobility may also be used as electrodes,electrolytes and sensors for electrochemical applications. Theoxygen-ions will move through such an electrolyte material under anapplied electrical field allowing the construction of oxygen pumps forcatalysis and other oxidising or reduction processes involving thesupply or extraction of atomic oxygen. The oxygen-ions will also movethrough such an electrolyte material under a concentration gradientallowing the construction of batteries, fuel cells and oxygen monitors.For these materials to act effectively as electrolytes in suchapplications it is necessary that they have high oxygen-ion mobilitythrough the atomic structure of the materials and at the same time havea low electronic conductivity so that the dominant current flow is byoxygen-ions and not electrons. For these materials to act effectively aselectrodes in such applications it is necessary that they have a highelectronic conductivity as well as a high oxygen-ion mobility so thatelectrons which are the current carried in the external circuit maycouple, to oxygen-ions which are the current carrier in the internalcircuit. Electrochemical cells including fuel cells, batteries,electrolysis cells, oxygen pumps, oxidation catalysts and sensors aredescribed in “Superionic Solids” by S Chandra (North Holland, Amsterdam1981).

[0004] Solid electrolytes, otherwise known as fast-ion conductors orsuperionic conductors have self diffusion coefficients for one speciesof ion contained within their crystalline structure ranging from 10⁻⁷ to10⁻⁵ cm²/sec. A diffusion coefficient of about 10⁻⁵ m²/sec is comparableto that of the ions in a molten salt and thus represents the upper limitfor ion mobility in a solid and is tantamount to the sublattice of thatparticular ion being molten within the rigid sublattice of the otherions present. Such high diffusion mobilities translate to electricalconductivities ranging from 10⁻² to 1 S/cm, the latter limitcorresponding to that commonly found in molten salts. The n=0 member ofthe series Bi_(2+e−x)Pb_(x)(Sr,Ca)_(n+1−s)Cu_(n)O_(2n+4+d) and varioussubstituted derivatives are identified as solid electrolytes with highoxygen-ion mobility. The n=1,2 and 3 members of the series may have highoxygen-ion mobility as well as high electron conductivity and thus arepotentially applicable as electrode materials.

[0005] The invention provides certain novel metal oxide materials whichexhibit superconductivity at low temperatures and/or which are useful insuch electrode, electrolyte, cell and sensor applications, or aselectrochemical catalysts.

[0006] In broad terms the invention comprises metal oxide materialswithin the formula

R_(n+1−u−s)A_(u)M_(m+e)Cu_(n)O_(w)  (1)

[0007] where n≧0 and n is an integer or a non-integer, 1≦m≦2, 0≦s≦0.4,0≦e≦4, and 2n+(1/2)≦w≦(5/2)n+4, with the provisos that u is 2 for n≧1, uis n+1 for 0≦n<1

[0008] and where

[0009] R and A are each any of or any combination of Y, La, Ce, Pr, Nd,Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Ca, Sr, Ba, Li, Na, K, Rb or Cs,

[0010] M is any of or any combination of Cu, Bi, Sb, Pb, Tl or any othertransition metal,

[0011] Cu is Cu or Cu partially substituted by any of or any combinationof Bi, Sb, Pb, Tl or any other transition metal

[0012] O is O or O partially substituted by any of N, P, S, Se, or F,

[0013] and wherein the structure of the materials is characterised bydistorted or undistorted substantially square planar sheets of CuO₂ whenn>0 and distorted or undistorted substantially square sheets of R forn>1.

[0014] excluding where M is Bi, R is Ca and Sr, A is Sr and Ca, and sand e are 0 and where n=1, the material Bi₂(Sr_(1−x)Ca_(x))₂CuO_(8−d)with 0≦x≦0.3 and where n=2 the material Bi₂(Sr_(1−x)Ca_(x))₃Cu₂O_(10−d)with 0.16≦x≦0.33

[0015] and excluding RBa₂Cu₃O_(7−d)

[0016] and excluding, where R is as above excluding Ca, Sr, Ba, Li, Na,K, Rb or Cs, the material having formula

RBa₂Cu₄O_(8−d).

[0017] The n=1,2,3,4,5, . . . materials have pseudo-tetragonalstructures with lattice parameters a, b and c given by 5.3 Å≦a,b≦5.5 Åand c=18.3±v+(6.3±v′)n Å where 0≦v,v′≦0.3. The n=0 material extends overthe solubility range 0≦e≦4 and has orthorhombic or rhombohedral symmetrywith lattice parameter c=19.1±v Å.

[0018] Preferred materials of the invention are those of formula (1)wherein m is 2 and R is Ca and R is predominantly Bi and having theformula

Bi_(2+e−x)L_(x)Ca_(n+1−u−s)A_(u)Cu_(n)O_(w)  (2)

[0019] where L is any of or any combination of Pb, Sb, or Tl, and0<x<0.4.

[0020] More preferred materials of the invention are those of formula(2) where n≧1 and 0≦e≦0.4 and having the formula

Bi_(2+e−x)L_(x)Ca_(n+y−1−s)Sr_(2−y)A_(z)Cu_(n)O_(2n+4+d)  (3)

[0021] and where 0≦z≦0.4, −2≦y≦2, and −1≦d≦1.

[0022] Materials of formula (3) of the invention wherein n is 3 have theformula

Bi_(2+e−x)L_(x)Ca_(2+y−s)Sr_(2−y)A_(z)Cu₃O_(10+d).  (4)

[0023] Preferably in the n=3 materials of formula (4) L is Pb and0≦x≦0.4 and −1≦y,d≦1. A may preferably be Y or Na and 0≦z≦0.4 andpreferably O, with preferably −0.5≦y≦0.5 and −1≦d≦1. Preferably d isfixed in a range determined by annealing in air at between 300° C. and550° C., or by annealing in an atmosphere at an oxygen pressure orpartial pressure and temperature equivalent to annealing in air atbetween 300° C. and 550° C. Most preferably 0.2≦e,s≦0.3, 0.3≦x≦0.4 and−0.1≦y≦0.1.

[0024] Especially preferred n=3 materials of the invention areBi_(1.9)Pb_(0.35)Ca₂Sr₂Cu₃O_(10+d), Bi_(2.1)Ca₂Sr₂Cu₃O_(10+d),preferably wherein d is fixed in a range determined by annealing in airat between 300° C. and 550° C., or by annealing in an atmosphere at anyoxygen pressure or partial pressure and temperature equivalent toannealing in air at between 300° C. and 550° C.

[0025] Materials of formula (3) of the invention wherein n is 2 have theformula

Bi_(2+e−x)L_(x)Ca_(1+y−s)Sr_(2−y)A_(z)Cu₂O_(8+d)  (5)

[0026] where −1≦d≦1.

[0027] Preferably in the n=2 materials of formula (5) L is Pb and 0<xand most preferably 0<x≦0.4, z is 0, and −1≦y,d≦1. A may preferably be Yor Na; where A is Y preferably 0<z≦0.4 and most preferably 0<z≦0.1 andwhere A is Na preferably 0<z≦0.4, x is 0, and −1≦y,d≦1; in both casespreferably d is fixed in a range determined by annealing in air atbetween 700° C. and 830° C. or in 2% oxygen at between 600° C. and 800°C., or by annealing in an atmosphere at an oxygen pressure or partialpressure and temperature equivalent to annealing in air at between 700°C. and 800° C.

[0028] A preferred n=2 material is that of formula (5) wherein L is Pb,and where 0<x≦0.4, z is 0, 0≦e,s≦0.25, y is −0.5, and d is fixed in arange determined by annealing the material in air between 600° C. and800° C., or by annealing in an atmosphere at an oxygen pressure orpartial pressure and temperature equivalent to annealing in air atbetween 700° C. and 800° C.

[0029] A further preferred n=2 material is that of formula (5) wherein Lis Pb, and where 0<x≦0.4, z is 0, 0≦e,s≦0.25, y is 0, and d is fixed ina range determined by annealing in air at between 700° C. and 830° C. orin 2% oxygen at between 600° C. and 800° C. or by annealing in anatmosphere and at an oxygen pressure or partial pressure and temperatureequivalent to annealing in air at between 700° C. and 830° C.

[0030] A further preferred n=2 material is that of formula (5) wherein Lis Pb, and where 0≦e,s≦0.25, 0≦x≦0.4, y is 0.5, z is 0, and d is fixedin a range determined by annealing in air at between 450° C. and 700° C.or by annealing in an atmosphere at an oxygen pressure or partialpressure and temperature equivalent to annealing in air at between 450°C. and 700° C.

[0031] A further preferred n=2 material is that of formula (5) havingthe formula Bi_(2.1+e)Pb_(0.2)Ca_(1−s)Sr₂Cu₂O_(8+d) and where 0≦e,s≦0.2and d is fixed in a range determined by annealing in 2% oxygen atbetween 770° C. and 830° C. or in an atmosphere at an oxygen pressure orpartial pressure and temperature equivalent to annealing in 2% oxygen atbetween 770° C. and 830° C.

[0032] A further preferred n=2 material is that of formula (5) havingthe formula Bi_(2+e)Ca_(1+y−s)Sr_(2−y)Cu₂O_(8+d) where −0.5≦y≦0.5 and0<e,s≦0.25 and most preferably wherein y is −0.5 or most preferably O,and d is fixed in a range determined by annealing in air at between 600°C. and 800° C. or by annealing in an atmosphere at an oxygen partialpressure and temperature equivalent to annealing in air at between 600°C. and 800° C.; of the above formula or wherein y is 0.5, and d is fixedin a range determined by annealing the material in air at between 450°C. and 700° C. or in an atmosphere other than air at an oxygen pressureor partial pressure and temperature equivalent to annealing in air atbetween 450° C. and 700° C.

[0033] Materials of formula (1) of the invention wherein n is 1 have theformula

Bi_(2+e−x)L_(x)Ca_(y−s)Sr_(2−y)A_(z)CuO_(6+d)  (6)

[0034] where 0≦y≦1 and −1≦d≦1

[0035] Preferably in the n=1 materials of formula (6) L is Pb and0<x<0.4. A is preferably Na and 0<z≦0.4.

[0036] A preferred n=1 material is that of formula (6) wherein L is Pb,and where 0<x≦0.4, 0≦e,s≦0.25 y is 0.7, and z is 0.

[0037] A further preferred n=1 material is that of formula (6) wherein Lis Pb, and where 0≦e,s≦0.25, 0≦x≦0.4, y is 1 and z is 0.

[0038] A further preferred n=1 material is that of formula (6) wherein Lis Pb, and where z is 0, 0≦e,s≦0.25 0.3≦x≦0.4, and 0.5≦y≦0.7.

[0039] A preferred n=1 material isBi_(1.85)Pb_(0.35)Ca_(0.4)Sr_(1.4)CuO_(6+d).

[0040] A preferred n=1 material is Bi_(2+e)Ca_(y−s)Sr_(2−y)CuO_(6+d)where 0<e,s≦0.25 and y is 1 and d is fixed in a range determined byannealing in air at between 300° C. and 500° C. or by annealing in anatmosphere at an oxygen pressure or partial pressure and temperatureequivalent to annealing in air at between 300° C. and 500° C.

[0041] A further preferred n=1 material is that of formula (6) where yis 0.67, and d is fixed in a range determined by annealing in air atbetween 400° C. and 600° C. or by annealing in an atmosphere at anoxygen pressure or partial pressure and temperature equivalent toannealing in air at between 400° C. and 600° C.

[0042] Materials of formula (1) of the invention where n is 0 have theformula

Bi_(2+e−x)L_(x)A_(1−z)A′_(z)O_(w)  (7)

[0043] where 0≦x≦0.4 and 0≦z≦1 with the proviso that z is not 0 when xis 0, and where 0≦e≦4.

[0044] Preferably in the n=0 materials of formula (7) wherein L is Pband preferably A is Ca, Sr, or Ba or any combination thereof. Where A isCa preferably A′ is Sr, Ba, Na or Y, or any combination thereof. Where Ais Sr preferably A′ is Ba, Na or K, or any combination thereof. Mostpreferably A is Ea and A′ is K.

[0045] Specific materials of the invention includeBi_(2+e)Sr_(0.8)Na_(0.2)O_(4+d), Bi_(2+e)Ba_(0.5)K_(0.2)O_(4+d),Bi_(2+e)Ca_(0.5)Sr_(0.5)O_(4+d), Bi_(2+e)Ba_(0.5)Sr_(0.5)O_(4+d),Bi_(1.9+e)Pb_(0.1)SrO_(4+d), and Bi_(1.9+e)Pb_(0.1)BaO_(4+d), with 0≦e≦4and 3≦d≦10.

[0046] The invention encompasses materials of formula (7) wherein n≧2,s=0 and M is Cu or Cu substituted by any of or any combination of Bi,Sb, Pb, Tl or any other transition metal, and having the formula

R_(n−1)A₂Cu_(m)Cu_(n)O_(w)  (8)

[0047] Preferably in materials of formula (8) of the invention A is Ba,m is 1 or 2, and n is 3, 4, . . . A material of this class may be wherem=3/2 and n=2, and having the formula

RBa₂Cu_(3.5)O_(7+d)

[0048] where −0.5≦d≦0.5, and where R is Y.

[0049] The invention encompasses a material having the formula

Y_(n−1)Ba₂Cu_(n+1)O_((5/2)n+3/2+d)

[0050] where −1≦d≦1 and n is 3,4,5, . . . ,

[0051] a material having the formula

Y_(n−1)Ba₂Cu_(n+2)O_((5/2)n+5/2+d),

[0052] and,

[0053] a material having the formula

RBa₂Ca_(n−2)Cu_(n+2)O_((5/2)n+5/2+d)

[0054] wherein n is 3,4,5, . . . preferably where R is Y.

[0055] The materials of the invention may be formed as mixed phase orintergrowth structures incorporating structural sequences from a numberof the above described materials also including sequences from thematerial RBa₂Cu₃O_(7−d) and its derivatives. For example, this includesRBa₂Cu_(3.5)O_(7.5−d) comprising, as it does, approximately alternatingsequences of RBa₂Cu₃O_(7−d) and RBa₂Cu₄O_(8−d). This also includesmaterials with general formula (1) with n taking non-integral valuesallowing for the fact that, for example, a predominantly n=2 materialmay have n=1 and n=3 intergrowths. This also includes materials withgeneral formula (1) with n taking non-integral values allowing forordered mixed sequences of cells of different n values, for example,n=2.5 for alternating sequences of n=2 and n=3 slabs.

[0056] Typically, the materials of the invention may be prepared bysolid state reaction of precursor materials such as metals, oxides,carbonates, nitrates, hydroxides, or any organic salt or organo-metallicmaterial, for example, such as Bi₂O₃, Pb(NO₃)₂, Sr(NO₃)₂, Ca(NO₃)₂ andCuO for BiPbSrCaCuO materials. The materials of the invention may alsobe prepared by liquid flux reaction or vapour phase depositiontechniques for example, as will be known to those in the art. Followingforming of the materials oxygen loading or unloading as appropriate toachieve the optimum oxygen stoichiometry, for example forsuperconductivity, is carried out. The above preparation techniques aredescribed in “Chemistry of High Temperature superconductors”—Eds. D LNelson, M S Whittingham and T F George, American Chemical SocietySymposium Series 351 (1987); Buckley et al, Physica C156, 629 (1988);and Torardi et al Science 240, 631 (1988), for example. The materialsmay be prepared in the form of any sintered ceramic, recrystallisedglass, thick film, thin film, filaments or single crystals.

[0057] In order to achieve maximum strength and toughness for thematerials, it is important that they are prepared to a density close tothe theoretical density. As prepared by common solid-state reaction andsintering techniques, densities of about 80% theoretical density canreadily be achieved. Higher densities may be achieved by, for example,spray drying or freeze drying powders as described for example inJohnson et al, Advanced Ceramic Materials 2, 337 (1987), spray pyrolysisas described for example in Kodas et al, Applied Physics Letters, 52,1662 (1988), precipitation or sol gel methods as described for examplein Barboux et al J. Applied Physics 63, 2725 (1988), in order to achievevery fine particles of dimension 20 to 100 mm. After die-pressing thesewill sinter to high density. Alternatively, to achieve higher densitiesone may hot press, extrude, or rapidly solidify the ceramic materialfrom the melt after solid state reaction or grow single crystals.

[0058] Preparation of the materials of the invention may be carried outmore rapidly if in preparation of the materials by solid state reactionof precursor material any or all of the cations in the end material areintroduced as precursors in the nitrate or hydroxide forms for rapidreaction of bulk material in the nitrate or hydroxide melt. Both thetemperature and duration of the preparation reaction may be lowered byusing nitrate or hydroxide precursors to introduce the cations. Meltingof the nitrate and/or hydroxide precursors allows intimate atomic mixingprior to decomposition and efflux of oxides of nitrogen.

[0059] After preparation the materials may be sintered or (re-)ground tosmall particles and pressed to shape and sintered as desired to form theend material for use, as is known in the art and/or annealed to relievestresses and increase strength and toughness as is similarly known inthe art for the unsubstituted materials. The materials after preparationmay as necessary be loaded on unloaded with oxygen to achieve theoptimum stoichiometry for superconductivity, optimised oxygen mobility,or other material properties. As stated, for n=2 BCSCO materials forexample this generally requires oxygen unloading into the materials andwith known technique this is generally carried out by annealing at 700°C. to 830° C. in air over 1 to 4 hours followed by rapid quenching intoliquid nitrogen, for example. Most suitably, oxygen loading or unloadingis carried out during cooling from the reaction temperatures immediatelyafter the preparation reaction, where the materials are prepared bysolid state reaction for example. Alternatively and/or additionallyoxygen loading may be carried out during sintering or annealing in anoxygen containing atmosphere at an appropriate pressure or partialpressure of oxygen. Without loss of generality the materials may beannealed, cooled, quenched or subjected to any general heat treatmentincorporating Ago or Ag₂O as oxidants or in controlled gaseousatmospheres such as argon, air or oxygen followed by rapid quenching soas to control the oxygen stoichiometry of the novel materials, the saidstoichiometry being described by the variables w or d. The materials maybe used as prepared without necessarily requiring oxygen loading orunloading for forming electrodes, electrolytes, sensors, catalysts andthe like utilising high oxygen mobility property of the materials.

[0060] The invention is further described with reference to thefollowing examples which further illustrate the preparation of materialsin accordance with the invention. In the drawings which are referred toin the examples:

[0061]FIG. 1 shows a graph of electrical conductivity as a function ofinverse temperature. Open circles are for heating and closed circles arefor cooling when measurements are made at 1592 Hz. The triangles referto the equivalent conductivity for the mid-frequency relaxation peak(upward pointing triangles) and the high-frequency peak (downwardpointing). Open triangles and the circles refer to Bi₂SrO₄ and the fulltriangles refer to Bi₂Sr_(0.9)Na_(0.1)O₄.

[0062]FIG. 2 shows Cole-Cole plots of Z″ versus Z′ measured at 644° C.for Bi₂SrO₄ in oxygen, then in nitrogen then in oxygen.

[0063]FIG. 3 shows Cole-Cole plots of Z″ versus Z′ measured at 736° C.for Bi₂SrO₄ in oxygen, then in nitrogen then in oxygen.

[0064]FIG. 4 shows V₁₂/ln(p₁/p₂) plotted against temperature where V₁₂is the potential developed over a concentration cell using Bi₂SrO₄ asthe electrolyte with oxygen partial pressures of p₁ and p₂ on eitherside of the cell.

[0065]FIG. 5 shows a plot of the conductivity measured at 1592 Hz as afunction of temperature for Bi₂Sr_(0.8)Ca_(0.5)O₄ (solid symbols) andfor Bi₂SrO₄ (open symbols).

[0066]FIG. 6 shows a plot of zero resistance T_(c) against the annealingtemperature in air from which the sample was quenched into liquidnitrogen. (X) Bi_(2.1)Ca₂Sr₂Cu₃O₁₀; (∘) Bi_(2.1)CaSr₂Cu₂O₅; (+)Bi_(2.1)Ca_(0.5)Sr_(2.8)Cu₂O₆; (Δ) Bi_(2.1)Ca_(1.8)Sr_(1.6)Cu₂O₈; ()Bi_(2.1)CaSr₂Cu₂O₈ quenched from 2% oxygen; (□)Bi_(2.1)Ca_(0.67)Sr_(1.33)CuO₆; (⋄) Bi_(2.1)CaSrCuO₆. Inset: plot ofmaximum T_(c) versus n the number of Cu layers.

[0067]FIG. 7 shows a plot of the XRD diffraction patterns forBi_(1.86)Pb_(0.35)Ca_(0.4)Sr_(1.4)CuO_(6+d) obtained using CoKaradiation.

[0068]FIG. 8 shows the XRD patterns for Pb-substituted compounds (a) n=2x=0.2 and (b) n=3 x=0.35.

[0069]FIG. 9 shows the temperature dependence of the resistivity forPb-substituted n=2 material x=0.2 (upper plot and x=0.3 (lower plot).

[0070]FIG. 10 shows the zero resistance T_(c) obtained as a function ofanneal temperature for the n=2 and n=3 unsubstituted (open symbols, x=0)and Pb-substituted samples (filled symbols). ∘:n=2, x=0.2, 21% oxygen;□:n=2, x=0.2 2% oxygen; Δ:n=2, x=0.2 0.2% oxygen; and ⋄:n=3, x=0.35, 21%oxygen.

[0071]FIG. 11 shows the temperature dependence of resistivity for (a)n=2 and x=0.2 reacted at 800° C. then annealed in air at 800, 700, 600and 500° C. before quenching into liquid nitrogen. A typical curve forthe unsubstituted x=0 material is shown in the inset; (b) for n=3 andx=0.35 with typical behaviour rot x=0 shown in the inset.

[0072]FIG. 12 shows the temperature dependence of the resistivity forBi_(2.1)CaSr₂Cu₂O₆ annealed in air at various temperatures shown in °C.(a) 5% Y substitution and (b) no substitution.

[0073]FIG. 13 shows a plot of the zero resistance T_(c) as a function ofair anneal temperature for 5% Y-substituted Bi_(2.1)CaSr₂Cu₂O₆ (solidhexagons). For comparison, the T_(c) values for unsubstituted n=1, n=2and n=3 are also shown (open data points).

[0074]FIG. 14 shows a plot of the temperature dependence of theresistivity for the nominal compositionBi_(1.9)Pb_(0.35)Ca_(0.9)Y_(0.1)Sr₂Cu₂O₆ annealed and quenched at 500°C., 700° C. and 820° C. showing no change in T_(c).

[0075]FIG. 15 shows a series of resistivity plots against temperaturefor n=3 and n=2 after annealing in air and then quenching into liquidnitrogen. The annealing temperatures are indicated in °C.

[0076]FIG. 16 shows the [5{overscore (5)}1] zone axis electrondiffraction patterns for n=1, n=2 and n=3 indexed on a 5.4 Å×5.4 Å×2 cÅcell where c=18.3+6.3nÅ.

EXAMPLE 1

[0077] (n=0)

[0078] Samples of composition Bi_(2+e)SrO_(4+d) with e=0, 0.25, 0.5,1.25 and 2 were prepared by solid-state reaction of Bi₂O₃ and SrCO₃ at740° C. in gold crucibles for 15 hrs. All samples were quenched from thefurnace into liquid nitrogen and investigated by x-ray diffraction,electron diffraction and SEM electron-beam x-ray analysis to confirmcompositions of crystallites. The e=0 material was white-yellow oforthorhombic structure with lattice parameters a=4.26 Å, b=6.10 Å andc=19.38 Å while the remaining solid-solution compositions were yellowwith rhombohedral symmetry and lattice parameters varying smoothly withcomposition given by a=9.852−0.09eÅ and α=23.28+0.2e°.

[0079]FIG. 1 shows the electrical conductivity measured at 1592 Hz forthe e=0 material. This shows a rapid rise in conductivity above 700° C.towards values typical of solid-state electrolytes. The rapid risecoincides with a broad endothermic DTA peak indicative of a diffusefast-ion transition. Complex impedance spectroscopy reveals threeseparate relaxation peaks: a broad low-frequency peak with effectivecapacitance in the microFarad range which is attributable tointer-granular impedance; and two higher frequency peaks attributable tooxygen-ion relaxation. The conductivities associated with these two highfrequency peaks are plotted as the open triangles in FIG 1. FIG. 2 showsCole-Cole plots at 644° C. in oxygen, then in nitrogen, then in oxygenwhile FIG. 3 shows the same sequence at 736° C. In nitrogen a distinctWarburg-type impedance appears with the characteristic 45° slope. Thisarises from the diffusive depletion of oxygen ions at the surface whenin an oxygen-free ambient and confirms the origin of the conductivity inoxygen-ion transport. A concentration cell was constructed using asintered pellet of e=0 material over which a difference in oxygenpartial pressure was maintained while the cell emf was measured. If thetransport coefficient is dominated by oxygen-ion mobility rather thanelectron transport then the emf V₁₂ is given by the Nernst equation

V ₁₂=(RT/nF)ln(p ₁ /p ₂)

[0080] Here R is the gas constant. T is temperature in degrees Kelvin, Fis the Faraday constant and n (=4) is the number or moles of electroncharge produced by one mole of O₂. p₁ and p₂ are the partial pressuresof oxygen on either side of the cell. FIG. 4 shows V₁₂/ln(p₁/p₂) plottedagainst T and the data follows the ideal theoretical line with slopeR/nF, thus confirming that the transport number is dominated by theoxygen-ion transport number.

EXAMPLE 2

[0081] (n=0)

[0082] Samples of composition Bi₂Sr_(1−z)Na_(z)O_(4+d) for z=0.1 andz=0.2 were prepared at 770° C. by reaction of the carbonates or Na andSr with Bi₂O₃ for 15 hrs. XRD showed that these were single phaseexhibiting the same orthorhombic structure as the unsubstituted e=0material. The conductivities associated with the two high frequencycomplex impedance relaxations are plotted as solid triangles and theseshow enhancements in conductivity above that for unsubstituted materialof 10 times and 30 times respectively.

EXAMPLE 3

[0083] (n=0)

[0084] Samples of composition Bi₂Sr_(1−z)Ca_(z)O_(4+d) for z=0.33 andz=0.5 were prepared at 770° C. by reaction of the carbonates of Ca andSr with Bi₂O₃ for 15 hrs. XRD showed that these were single phasematerials exhibiting the same hexagonal structure as occurs in theunsubstituted e>0 solid-solubility range. Lattice c-parameters obtainedwere 19.14 Å (z=0.33) and 19.06 (z=0.5). In addition, electrondiffraction reveals the existence of a superstructure in the basal planeas evidenced by hexagonal satellites encircling each principaldiffraction spot. For z=0.33 the superstructure length is 2.76 times thea-parameter. FIG. 5 shows conductivity measured at 1592 Hz for z=0.5(solid symbols) and this is compared with the unsubstituted z=0conductivity (open symbols) and Y-stabilised zirconina. Over much of therange the conductivity to enhanced by Ca-substitution by a factor of 100times and is comparable to Y-zirconia.

EXAMPLE 4

[0085] (n=0)

[0086] Samples of composition Bi_(2+x−y)Pb_(y)SrO_(w) with 0≦x≦0.2 andy=0.2 and y=0.4 were prepared by stoichiometric reaction of Bi₂O₃, SrCO₃and PbO at 770° C. and quenched into liquid nitrogen producing a brightyellow sintered material.

[0087] The structure as prepared was predominantly orthorhombic with thesame structure as the unsubstituted material. When annealed and quenchedfrom 500° C. the rhombohedral structure of the unsubstitutedsolid-solution was adopted and the colour became brown/green. Additionalphases were not evident indicating complete substitution at the y=0.4level. Impedance spectroscopy showed that the mid- and high-frequencyrelaxation peaks overlapped, producing single broadened relaxation peak.The conductivity associated with this peak measured during heating ofthe as prepared material increased with Pb-substitution but on coolingthe conductivity remained high.

EXAMPLE 5

[0088] (n=1)

[0089] Samples corresponding to the starting compositionBi₂Sr_(2−x)Ca_(x)CuO_(6+d) were prepared by reacting carbonates of Srand Ca with the oxides of Bi and Cu at temperatures between 780 and 830°C. for periods of time ranging from 1 to 12 hrs. The n=1 phase ispromoted by short reaction times (1 to 2 hrs), temperatures near 80° C.and lower calcium content. We could not prepare the calcium-pure phasex=2, but nearly single-phase material was obtained for s=0, 0.67 and1.0. The structure was pseudotetragonal with lattice parameters,respectively of a=5.414 Å and c=24.459 Å, a=5.370 Å and c=24.501 Å, anda=5.370 Å and c=24.287 Å. For s=0.67 and s=1.0 incommensuratesuperlattice structures in the b-direction were observed with dimensionranging from 4.3 to 5.3 times the a-parameter.

[0090] Samples were annealed to equalibrium oxygen stoichiometry atvarious temperatures in air, then rapidly quenched from the furnace intoliquid nitrogen. The DC electrical resistivity was measured with a fourterminal technique and the zero-resistance T_(c) is plotted in FIG. 6with squares and diamonds as a function of anneal temperature. T_(c)evidently passes through a maximum corresponding to the optimum oxygenstoichiometry.

[0091] The true c-axis length may be twice the figure quoted above dueto a two-times c-axis superstructure. Electron diffraction patternsinterpreted as [5{overscore (5)}1] zone-axis patterns may be uniquelyindexed on a 5.4 Å×5.4 Å×49 Å unit cell. The same applies to all othern=1,2 and 3 electron diffraction patterns investigated suggesting ageneral unit cell of 5.4 Å×5.4 Å×2c Å where c is approximately18.3+6.3nÅ.

EXAMPLE 6

[0092] (n=1)

[0093] Samples of nominal compositionBi_(1.6)Pb_(0.2)Sr_(1.3)Ca_(0.7)CuO_(6+d),Bi_(1.6)Pb_(0.2)SrCaCuO_(6+d), andBi_(1.9)Pb_(0.86)Sr_(1.3)Ca_(0.7)CuO_(6+d), were prepared by reactingstoichiometric proportions of Bi₂O₃, Pb(NO₃)₂, Sr(NO₃)₂, Ca(NO₃)₂ andCuO. The n−1 materials are favoured by short reaction times and as aconsequence the nitrates are advantageous as they allow homogeneous andrapid mixing in the nitrate melt. The precursor materials were milled,pressed into pellets and reacted at 800° C. for 15 minutes, remilled,pressed into pellets and sintered for 1 hr at the same temperature. Theresultant materials were very nearly single phase n=1 materials. Thefirst of the compounds listed above was apparently tetragonal withparameters a=5.355 Å and c=24.471, but this material and even morepredominantly the third material with x=0.35 and y=0.7 showed thepresence of particles of compositionBi_(1.86)Pb_(0.38)Sr_(1.4)Ca_(0.4)CuO_(6+d). By reacting precursors ofthis composition good phase-pure material was obtained as shown by FIG.7. Impurity peaks are indicated by dots. This material is orthorhombicwith parameters a=5.313 Å, b=5.391 Å and c=24.481 Å, and, moreover it issemiconducting.

EXAMPLE 7

[0094] (n=2)

[0095] Samples of nominal compositionBi_(2.1)Sr_(2−y)Ca_(1+y)Cu₂O_(6+d), with y=−0.5, −0.25, 0 and 0.5 wereprepared using stoichiometric proportions of the carbonates of Sr and Caand the oxides of Bi and Cu reacted at temperatures between 860 and 870°C. for 8 to 15 hrs. The resulting material was ground, milled, pressedinto pellets and sintered for another 8 to 15 hrs at 860 to 870° C. inair. This procedure produced very nearly single-phase material with asystematic variation in lattice parameters as shown in the table belowconfirming the intersubstitution of Sr and Ca. As lattice parameters aredependent upon oxygen stoichiometry determined by annealing temperatureand ambient oxygen partial pressure, all these XRD measurements werecarried out on material quenched into liquid nitrogen after annealingfor up to 12 hrs at 400° C. in air. y a c −0.5 5.415 30.908 −0.25 5.41030.894 0 5.405 30.639 0.5 5.402 30.683

[0096] The structure is centred pseudotetragonal as described bySubramanian et al for the y=0 material in Science 239, 1015 (1988).Electron diffraction shows that a 4.75 times incommensuratesuperstructure exists in the b-direction and as indicated in example 5there may be a two times superlattice in the c-direction as indicated bythe [5{overscore (5)}1] zone axis electron diffraction pattern.

[0097] As discussed in example 5 the zero-resistance T_(c) was measuredfor samples annealed at various temperatures and oxygen partialpressures. The data for anneals in air is plotted in FIG. 6 for they=0.5, y=0 and y=0.5 samples by plusses, circles and squaresrespectively. Again T_(c) maximises at an optimum oxygen stoichiometryfor each composition. The solid circles are obtained for anneals of they=0 material in 2% oxygen and the displacement of the curve confirmsthat the optimisation is associated with oxygen stoichiometry. We findthat the lattice c-parameter varies with anneal temperature but for twodifferent pairs of oxygen partial pressure and anneal temperature whichgive the same T_(c) the lattice parameters are also the same.

EXAMPLE 8

[0098] (n=2)

[0099] Samples of composition Bi_(2.2−x)Pb_(x)CaSr₂Cu₂O_(6+d) with x=0,0.1, 0.2. 0.3, 0.4 and 0.5 were prepared by solid state reaction ofBi₂O₃, CaCO₃, SrCO₃, CuO and PbO for 12 hours at temperatures between850 and 865° C. The samples were ground, pressed and sintered at thesame temperature for a further 12 hours. FIG. 8 shows the XRD trace forx=0.2 which confirms nearly single phase material. Minor impurity peaksare marked hy dots. Pb substitution is confirmed by observing asystematic variation in lattice parameters with x, as follows

a=6.405−0.048 xÅ

[0100] and

c=30.830−0.094 xÅ

[0101] Electron beam x-ray analysis of crystallites also confirmed theabove compositions. Electron diffraction shows that the b-axissuperstructure remains at about 4.75X for 0≦x≦0.2. For 0.2≦x≦0.35 thissuperstructure contracts to 4.5X and a second b-axis superstructureappears with length 7.3X. Substitution for x>0.35 did not occur underthe conditions of preparation. Samples were annealed at varioustemperatures at oxygen partial pressures of 2.1×10⁴ Pa (air), 2×10⁵ Paand 2×10² Pa then quenched into liquid nitrogen. The DC resistivity ofthese samples was measured using a 4-terminal method and ACsusceptibility was also measured. FIGS. 9a and 9 b show the resistivitycurves for anneals in air for x=0.2 and x=0.3 respectively. T_(c) isseen to decrease with decreasing anneal temperature. FIG. 10 shows thezero resistance T_(c) versus annealing temperature for the three oxygenpartial pressures for both x=0 and x−0.2. T_(c) is seen to pass througha maximum for an optimum oxygen content. The maximum T_(c) obtained is93 K. This increase is not due to n=3 material which has a differentbehaviour also shown in FIG. 10. The optimised T_(c) is maximised at 93K for x=0.2 and falls 4 K for x=0.3.

[0102] The sharpness of the resistive transitions should be noted inFIG. 9 and compared with the typical best curve obtained for x=0 shownin the inset in FIG. 11a which exhibits a typical resistive tail. Theresistivity curves shown in the main part of FIG. 11a are for a x=0.2sample prepared at the lower temperature of 800° C. The variation inT_(c) as a function of anneal temperature is similar to that shown inFIG. 9a but the normal state resistivity varies differently. Electronmicroprobe analysis indicated crystallites which were Pb-rich anddeficient in Sr and Ca indicating substitution of Pb on the alkali-earthsites.

EXAMPLE 9

[0103] (n=2)

[0104] Samples of composition Bi_(2.1)Ca_(1−x)R_(x)Sr₂Cu₂O_(6+d) wereprepared by solid state reaction of Bi₂O₃, CaCO₃, SrCO₃, CuO and R₂O₃where R is Y or one of the rare earth elements. Compositions with x=0,0.05, 0.1, 0.2, 0.4, 0.9 and 1.0 were reacted at temperatures rangingfrom 860° C. to 900° C. as the rare earth content was increased. Sampleswere investigated by x-ray diffraction, electron diffraction, IRspectroscopy, thermal gravimetry, and the temperature dependence ofresistivity and AC susceptibility. In the following example we dealexclusively with the results for Y-substitution.

[0105] The end member x=1 was XRD phase pure and notably has the samestructure as Bi_(2.1)CaSr₂Cu₂O_(a+δ) except that the symmetry is reducedfrom tetragonal to orthorhombic as shown by the splitting of the (200)XRD peak. For Bi_(2.1)YSr₂Cu₂O_(a+δ), annealed in air at 400° C. thelattice parameters are a=5.430 Å, b=5.473 Å and c=30.180 Å. Electrondiffraction also reveals the presence of an 8X incommensuratesuperstructure, of 43.5 Å in the b-direction.

[0106] Attempts to study the metal to insulator transition atintermediate substitutions (0<x<1) were prevented by the absence of asubstitutional solubility range under the conditions of preparation. Inthis intermediate range samples were a mixed phase of the x=0 and x=1end-members except at compositions close to x=0 and x=1 where dopingappears to be possible. Interestingly, Y substitution for Ca at the 5%level in Bi_(2.1)CaSr₂Cu₂O_(6+d) appears to raise T_(c). FIG. 12a showsthe temperature dependence of the resistivity for such a sample annealedand quenched at different temperatures in order to vary the oxygenstoichiometry, d. This may be compared with the same data shown in FIG.12b for the x=0 unsubstituted n=2 material. The substitution has bothsharpened the resistive transition and raised the zero resistance T_(c).Otherwise the overall behaviour is similar and quite distinct from then=3 behaviour.

[0107] The zero-resistance T_(c) is plotted in FIG. 13 as a function ofanneal temperature (solid data points) and evidently T_(c) is maximisedat >101 K. FIG. 13 also includes T_(c) data for unsubstituted n=1, n=2and n=3 material for comparison. Like unsubstituted n=2 theY-substituted material appears to exhibit a maximum T_(c) for anneals inair above 820° C., However, above this temperature the effects ofannealing and quenching are greatly modified by the proximity of a phasetransition. In order to achieve maximum T_(c) anneals at an oxygenpartial pressure lees than that of air is required. The highest zeroresistance T_(c) we have observed in this system is 102 K. The elevatedT_(c) does not arise from the presence of n=3 material for severalreasons.

[0108] i) We are able to prepare single-phase n=3 material byPb-substitution for Bi. Attempts to substitute Y in this material at the5% level drives the reacted material completely to the n=2 phasetogether with the binary Ca₂CuO₃. We would therefore hardly expect Ysubstitution of the n=2 material to promote n=3 material.

[0109] ii) The annealing behaviour of T_(c) is similar to that forunsubstituted n=2 material with maxima occurring at 820° C. or higher.The maximum T_(c) for n=3 occurs for anneals at ˜400° C. and for annealsat 820° C. the n=3 T_(c) is as low as 80 K.

[0110] Particle by particle analyses by SEM electron beam x-ray analysisindicates that Ca remains fixed at one per formula unit while Sr isslightly depleted. This suggests Y substitution on the Ca-siteaccompanied by Ca substitution on the Sr-site with the formulaBi_(2.1)(Ca_(0.95)Y_(0.05))(Sr_(1.95)Ca_(0.0δ))Cu₂O_(δ). Startingcompositions appropriately depleted in Sr indeed offered the bestresistive transitions around 100 K with a minimal tail.

[0111] It may be that the substitutional solubility tends to occur onlyat grain boundaries as the sharp resistive transition to zero at 101 Kis accompanied by only a small diamagnetic signal in the ACsusceptibility commencing at ˜99 K. A sharp fall does not commence until˜95 K at which point the diamagnetic signal is only about 5% of itsfully developed value. The yttrium should therefore be dispersed moreuniformly throughout a sample by reaction of nitrates or by meltprocessing.

[0112] This example and particularly the chemical formula deduced forthe active phase responsible for raising T_(c) is presented by way ofexample without loss of generality. Because of the small diamagneticsignal appearing at ˜100 K it may be that the active phase is of anothercomposition and structure incorporating Bi/Ca/Y/Sr/Cu/O.

EXAMPLE 10

[0113] (n=2)

[0114] Predominantly single phaseBi_(1.9)Pb_(0.36)Ca_(0.9)Y_(0.1)Sr₂Cu₂O_(aδ) was prepared by solid statereaction or a pressed disc of Bi₂O₃, PbO, CaCO₅, SrCO₃, Y₂O₃ and CuO at860° C. for 12 hours. By annealing in air at various temperatures thenquenching into liquid nitrogen, the normal state resistivity is observedto change as shown in FIG. 14. However, the zero-resistance T_(c) doesnot change, remaining at just over 90 K for anneals at 500, 700 and 820.This is uncharacteristic of the parent n=2 superconductor as shown inFIG. 12 which required anneals at an oxygen partial pressure less thanthat of air in order to optimise T_(c) at just over 90 K. The combinedPb- and Y-substitution therefore simplifies the processing requirementsfor n=2 material.

EXAMPLE 11

[0115] (n=3)

[0116] A sample of nominal composition BiSrCaCu₅O_(x) was prepared fromthe carbonates of Sr and Ca, CuO and bismuth oxycarbonate by reacting at820° C. for 9 hrs, then for 10 hrs at 850° C. then for 10 hrs at 860° C.followed by air-quenching from the furnace. The sample was then annealedin air at temperatures ranging between 400° C. and 800° C. and quenchedfrom the furnace into liquid nitrogen. Four terminal electrical DCresistivity and the AC susceptibility was measured for each annealtemperature. FIG. 15 shows the resistivity curves obtained for thissample after each anneal. The resistivity drop which occurs around 110 Kis extrapolated to zero and the deduced tero resistances T_(c) is seento be maximised at 105 K for anneals at about 400° C. The annealingbehaviour is seen to be quite different from that of the n−2 material.This sample was pulverised, ground and investigeted by XRD. SEM energydispersive analysis of x-rays (EDX) and TEM electron diffraction. TheEDX analyses indicated a high proportion (>70%) of particles with atomicratios Bi:Sr:Ca:Cu of 2:2:2:3 though many of these particles showed Cucontents more like 2.8 to 2.9 indicating the occurrence of n=2intergrowths in the n=3 material. Like the n=2 material, crystals of n=3are platey and under TEM electron diffraction were found to exhibit a5.4 Å×5.4 Å subcell in the basal plane with the same 19/4 timesincommensurate superlattice structure in the b-direction. Thediffraction pattern for the [5{overscore (5)}1] zone axis shown in FIG.16 can be indexed on a 5.4 Å×5.4 Å×74 Å cell suggesting a sub-cellc-axis of 37 Å with a superstructure which doubles the c-axis. XRDpowder diffraction of this sample showed a broad basal reflectioncorresponding to a c-repeat of about 18 Å. This leads to the naturalconclusion that Bi₂Sr₂Ca₂Cu₃O₁₀ is structurally related toBi₂Sr₂Ca₁Cu₂O₆ by the insertion of an extra pair of Ca—CuO₂ sheets perunit sub-cell.

EXAMPLE 12

[0117] (n=3)

[0118] Samples of composition Bi_(2.2−x)Pb_(x)Ca₂Sr₂Cu₃O_(10+d) wereprepared by reaction of the oxides of Bi and Cu and the nitrates of Pb,Ca and Sr in stoichiometric proportions for 36 hrs at 860 to 865° C. inair. The XRD pattern shown in FIG. 8b indicates nearly single phasepseudo-tetragonal material with lattice parameters a=5.410 Å andc=37.125 Å. Like the n=2 x=0.2 material, electron diffraction indicatesa 4.5 times and a 7.3 times b-axis superlattice structure. The effect onresistivity curves of annealing in air at various temperatures is shownin FIG. 11b and the curve for the x=0 material is shown in the inset.The long resistive tail in the unsubstituted material is removed byPb-substitution. The effect of annealing temperature in air on the zeroresistance T_(c) is shown in FIG. 10 by the diamond shaped points forx=0.35 and x=0.

[0119] The foregoing describes the invention including preferred formsand examples thereof. The preparation of derivative materials and formsother than sintered ceramic form, ie, thin films, thick films, singlecrystals, filiaments and powders other than those specificallyexemplified will be within the scope of those skilled in the art in viewof the foregoing. The scope of the invention is defined in the followingclaims.

What is claimed is:
 1. A metal oxide material having the formulaB_(x)Sr₂Ca₂Cu₃O_(10+d) where B is Bi partially substituted by Pb and xis about 2.1.
 2. A metal oxide material as set forth in claim 1 whereind is fixed by annealing said metal oxide material at a temperature of300-550 C. in air or at an oxygen pressure or partial pressure andtemperature equivalent to annealing in air within said temperaturerange.
 3. A metal oxide material as set forth in claim 2 wherein d isfixed by annealing said metal oxide material at a temperature of about500 C. in air followed by quenching in liquid nitrogen.
 4. A metal oxidematerial having the formula (Bi,Pb)_(2.1)Sr₂Ca₂Cu₃O_(10+d) wherein Bi ispartially substituted by Pb.
 5. A metal oxide material as set forth inclaim 4 wherein d is fixed by annealing said metal oxide material at atemperature of 300-550 C. in air or at an oxygen pressure or partialpressure and temperature equivalent to annealing in air within saidtemperature range.
 6. A metal oxide material as set forth in claim 5wherein d is fixed by annealing said metal oxide material at atemperature of about 500 C. in air followed by quenching in liquidnitrogen.
 7. A metal oxide material having the formulaBi_(2.2−x)Pb_(x)Sr₂Ca₂Cu₃O_(10+d) wherein: 0<x≦0.4.
 8. A metal oxidematerial as set forth in claim 7 wherein: 0.15≦x≦0.4.
 9. A metal oxidematerial as set forth in claim 8 wherein: 0.30≦x≦0.4.
 10. A metal oxidematerial as set forth in claim 9 wherein x is about 0.35.
 11. A metaloxide material as set forth in claim 7 wherein d is fixed by annealingsaid metal oxide material at a temperature of 300-550 C. in air or at anoxygen pressure or partial pressure and temperature equivalent toannealing in air within said temperature range.
 12. A metal oxidematerial having the composition (Bi,Pb)_(2+x)(Sr,Ca)₄Cu₃O_(10+δ), wherex is about 0.1, the Pb/Bi ratio is less than 1, and the Ca/Sr ratio isabout
 1. 13. A metal oxide material according to claim 12 wherein−1≦δ≦1.
 14. A metal oxide material as set forth in claim 12 wherein thevalue of δ is fixed by annealing said metal oxide material at atemperature of 300-550 C. in air or at an oxygen pressure or partialpressure and temperature equivalent to annealing in air within saidtemperature range.